1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type or an electrostatic recording type, such as a copying machine or a laser printer, and, more particularly, to an image forming apparatus which forms an output image by transferring a visible image formed on an image carrier onto a recording medium carried by a recording medium carrier.
2. Related Background Art
FIG. 8 shows an example of a conventional electrophotographic color image forming apparatus. This color image forming apparatus includes a photosensitive drum 1 as an image carrier which is arranged so as to be rotatable in a direction indicated by an arrow in FIG. 8. Around the photosensitive drum 1, there are disposed a charger 2 (in this example, a corona charger) for evenly charging the surface of the photosensitive drum 1, an image exposing means 3 for forming an electrostatic latent image corresponding to image information on the photosensitive drum 1, a developing unit 400 for developing the electrostatic latent image formed on the photosensitive drum 1 into a visible image, a transfer unit 5 including a transfer drum 5a as a recording medium carrier, and a cleaner 6 for removing the developing agents remaining on the photosensitive drum 1.
In this conventional example, the image exposing means 3 is a laser beam exposing unit consisting of, e.g., a polygon mirror and a lens system. That is, a reflected light image obtained from an original scanner (not shown) is modulated with color image signals which are color-separated to have their respective colors by, e.g., a color separation filter., and is output as light images from a laser output unit. The laser beam exposing unit radiates these light images or light images E corresponding to these light images onto the photosensitive drum 1 (with a diameter of, e.g., 82 mm), forming electrostatic latent images corresponding to the color image signals of their respective colors.
In this conventional example, a rotary developing unit is also used as the developing unit 400. This rotary developing unit mounts four developing devices, e.g., a black developing device 400K, a cyan developing device 400C, a magenta developing device 400M, and a yellow developing device 400Y, on a rotating member 400a which rotates about a central shaft 400b. The rotary developing unit rotates a desired developing device to a development position opposing the photosensitive drum 1 and develops the latent image formed on the photosensitive drum 1.
The overall operation sequence of this color image forming apparatus will be briefly described below by taking a full-color mode sequence as an example. First, the photosensitive drum 1 is evenly charged by the charger 2. Subsequently, the image of an original is exposed by a laser beam which is modulated with, e.g., a cyan image signal, and the resulting electrostatic latent image is formed on the photosensitive drum 1. This latent image is developed by the cyan developing device 400C which is moved to the development position in advance, and the corresponding visible image (toner image) of cyan is formed by a toner consisting primarily of a resin on the photosensitive drum 1.
On the other hand, a recording medium such as a transfer sheet is supplied from a recording medium cassette 7a, 7b, or 7c (the recording medium is also fed manually in some cases) through a paper path indicated by a dotted line in FIG. 8 by a conveyor system constituted by pickup rollers, paper feed guides, paper feed rollers, and the like. The recording medium is wound around the transfer unit 5 in synchronism with a predetermined timing. In this conventional example, the transfer unit 5 includes the transfer drum 5a (with a diameter of, e.g., 164 mm) as a recording medium carrier, a transfer corona charger 5b for transferring the toner image formed on the photosensitive drum 1 onto the recording medium, an attraction corona charger 5c as an attraction charging means for attracting the recording medium to the transfer drum 5a, an attraction (contact) roller 5g serving as a counter electrode, an inner corona charger 5d, and an outer corona charger 5e. A recording medium carrier sheet 5f as recording medium carrying means consisting of a dielectric film is cylindrically, integrally looped around the circumferential opening of the transfer drum 5a which is axially supported to be rotatable.
The transfer drum 5a is rotated in a direction indicated by an arrow in FIG. 8 in synchronism with the photosensitive drum 1. In the transfer section, the cyan toner image developed by the cyan developing device 400C is transferred by the transfer charger 5b to the recording medium carried on the recording medium carrier sheet 5f. The transfer drum 5b keeps rotating to prepare for the transfer of an image of the next color (e.g., magenta).
The cleaner 6 removes substances, such as the residual toner, deposited on the photosensitive drum 1 from which the toner image is transferred. Thereafter, the photosensitive drum 1 is again charged evenly by the charger 2 and subjected to the above image exposure using a laser beam modulated with the next magenta image signal. During the exposure, the developing unit 400 rotates to move the magenta developing device 400M to the predetermined development position, thereby performing a predetermined magenta development.
In the transfer section, the resultant magenta toner image is transferred by the transfer charger 5b to the recording medium carried on the recording medium carrier sheet 5f. Consequently, the magenta toner image overlaps the cyan toner image. The transfer drum 5a keeps rotating to prepare for the transfer of an image of the next color (e.g., yellow).
Subsequently, the above process is similarly performed for the formation and the transfer of yellow and black images. When the overlapping transfer of the four color toner images is completed, the recording medium is discharged by a separating charger 5h and separated from the transfer drum 5a by a separating means 8 such as separating grippers. The recording medium thus separated is conveyed to a fixing unit (in this example, a heat roller fixing unit) 9 by an appropriate conveying means along the paper path indicated by the dotted line in FIG. 8. The recording medium is fixed at one time and delivered onto a tray 10 outside the apparatus. When the whole full-color print sequence is ended as described above, a predetermined full-color print image is formed.
The conventional image forming apparatus with the above arrangement, however, has the following drawbacks.
Since the developing unit is of a rotary type, it takes much time to cause the developing unit to rotate and move a desired developing device to the development position, resulting in time-consuming image formation.
For example, when two sheets of A4-size or letter-size (LTR) as a standard recording medium are attracted onto the recording medium carrier sheet 5f of the transfer drum 5a, as is well known, spacings between the sheets are narrowed. In many instances, therefore, a latent image to be developed which is formed on the photosensitive drum has already passed through the development position when a corresponding developing device of the developing unit is moved to the development position; i.e., the movement of the developing device is too slow. Consequently, development cannot be performed until the transfer drum rotates one more time, so the photosensitive drum must also perform an unnecessary extra rotation. This undesirably prolongs the time from the start to the completion of the image formation.
In addition, if the developing unit includes developing devices using two-component developing agents (to be described later), toner particles separated from carrier particles are liable to scatter. In this case, in the rotary developing unit, toner particles scatter from a developing device moved downward in the direction of gravity, contaminating the interior of the image forming apparatus.
Furthermore, since the diameter of the photosensitive drum is small, the photosensitive drum is readily damaged by transfer charging several times. That is, a drum (to be abbreviated as an OPC drum hereinafter) using an organic photoconductor (OPC) as a photosensitive body is generally used as the photosensitive drum for the industrial reason that the OPC is inexpensive and harmless. This photosensitive body is once negatively charged and then positively charged in performing transfer. If the positive transfer charging is performed with no recording medium, the photosensitive body transits to the positive potential side under the influence of the positive charge.
Generally, if the OPC drum which is to be used on the negative charging side is positively charged, it becomes difficult for the drum to return to the negative potential side. This results in unstable charging and unstable image formation after that.
For the reasons described above, there has been proposed an image forming apparatus using a large-diameter photosensitive drum and fixed developing devices.
An example of a conventional electrophotographic color image forming apparatus with this arrangement will be described below with reference to FIG. 9. Note that the same reference numerals as in the color image forming apparatus shown in FIG. 8 denote the same members, parts, and elements in FIG. 9, and a detailed description thereof will be omitted.
In the apparatus illustrated in FIG. 9, a photosensitive drum 1 has a large diameter. Around this photosensitive drum 1, cyan, magenta, yellow, and black developing devices 500C, 500M, 500Y, and 500K are arranged at fixed positions in the direction of rotation of the photosensitive drum 1. Note that the developing agent contained in each developing device is a two-component developing agent.
This conventional image forming apparatus shown in FIG. 9, however, has the following problems.
That is, in the apparatus shown in FIG. 9, scattering of toner particles to some extent is unavoidable even though the extent of scattering is smaller than that in the conventional rotary developing unit mentioned earlier; i.e., toner particles scatter to some extent even for a developing unit arranged at a fixed position. If toner particles scatter from the cyan developing device 500C on the upstream side, then these scattering toner particles flow to the downstream side along the rotating direction of the photosensitive drum 1 and are mixed in the magenta, yellow, and black developing devices 500M, 500Y, and 500K. Likewise, toner particles scattering from the magenta developing device 500M flow to the downstream side and are mixed in the yellow and black developing devices 500Y and 500K. This color mixing is notable especially when two-component developing agents are used as in this example.
In the developing device in which the colors are mixed as described above, e.g., in the yellow developing device, the toner components of magenta and cyan are mixed. The resultant toner is deposited on a yellow color-separated latent image during development. Consequently, an imperfect image in which colors are mixed is formed. This similarly occurs in the magenta developing device as another developing device. However, no such inconvenience as in the yellow and magenta developing devices takes place in the black developing device on the most downstream side. This is so because, even if the cyan, magenta, and yellow toner components are mixed in the black toner, this color mixing is not so conspicuous.
More specifically, since the visual sensitivity of black is high, no serious problem arises even if the toner components of cyan, magenta, and yellow with a lower visual sensitivity are mixed in with the black toner.
Another drawback Of the conventional image forming apparatus illustrated in FIG. 9 is as follows.
As shown in FIG. 10, when an A3-size recording medium (recording paper) P is fed to a transfer drum 5a, a portion with a length a [mm] in which no recording paper P exists is present on the transfer drum 5a. To transfer toner images of the respective colors to one A3-size recording sheet in this condition, the toner images are formed in sequence from the cyan developing device 500C on the most upstream side to those on the downstream side in an order of the magenta developing device 500M to the yellow developing device 500Y to the black developing device 500K. The toner images thus formed are sequentially transferred to the recording paper P. This toner image transfer is executable without unnecessarily idling the transfer drum 5a, since switching between the developing devices can be performed while the transfer drum 5a rotates a distance of a [mm].
If, however, the second recording paper is continuously fed in a continuous copying operation, then there is a distance of only a [mm] between the trailing end of the first recording paper to the leading end of the second recording paper. In this case, it is necessary to perform switching from the black developing device 500K to the cyan developing device 500C. However, the distance between the black and cyan developing devices 500K and 500C is b [mm] which is longer than the distance ba [mm]. For this reason, the leading end of the first cyan latent image of the second recording paper has already passed through the cyan developing device 500C when the trailing end of the last latent image of the first recording paper passes through the black developing device 500K.
In the above sequence, therefore, feed of the second recording paper is normally performed after the transfer drum 5a is idled once, thereby performing the development of the cyan latent image first. Consequently, the copying speed is lowered by this one idling of the transfer drum, and so degradation in mechanical performance is unavoidable.
If an independent developing motor is incorporated in each individual developing device so that in switching from the black developing device to the cyan developing device, the cyan developing device is operated to develop the cyan latent image on the second recording medium while the black developing device is developing the black latent image, continuous copying can be performed without idling the transfer drum.
In the above development system using the two-component developing agents, however, the torque of each developing device is large, and so a large-torque motor must be used to rotate the developing sleeve. The use of four such large-torque motors leads to a large increase in the manufacturing cost of the whole apparatus.
If a single developing device driving motor is used to rotate the developing sleeves of the individual developing devices via clutches of the respective colors, a large increase in cost can be avoided. However, since the torque of each developing device is large as described above, a very large torque variation occurs if the cyan developing device is driven during the development of the black latent image. This results in an uneven development in particularly the black latent image. Therefore, it is impossible to put this system into practical use.